Electronic temperature control

ABSTRACT

An electronic temperature control circuit is disclosed for use with swimming pool and spa heaters. The circuit employs a thermistor for sensing water temperature, and provides temperature control with a narrow differential. Safety circuits are also employed to prevent injury to the user or to the heater in the event of a component failure. The temperature control circuit is provided with means for presetting two desired temperature settings and for selecting one of the preset temperature settings for control. Remote control circuits are also disclosed for use with the temperature control circuit which permit temperature selection, temperature setting and a visual indication of heater status from a location remote from the heater.

This application is a division of application Ser. No. 184,638, filedSept. 8, 1980, now U.S. Pat. No. 4,361,274.

BACKGROUND OF THE INVENTION

This invention relates to electronic temperature controls and, moreparticularly, to electronic temperature controls for use with swimmingpool and spa heaters.

Temperature controls employing mechanical control means have been usedfor many years to control the water temperature in swimming pool and spaheaters. A typical mechanical control employs a sealed temperaturesensing metal bulb which is filled with a thermally expansive liquid.This bulb is connected to the actuating diaphragm of a pressure operatedelectrical switch through a metal capillary tube. The electrical switchin turn is connected to operate a heater controller such as an electricgas valve. In operation, the metal bulb is placed in close thermalcontact with the water entering the heater. As the water temperature isincreased by the heater, the fluid within the metal bulb expands in thecapillary tube, exerting a pressure on the diaphragm of the electricalswitch which is proportional to water temperature. The diaphragm in turnis biased against the capillary pressure by a spring. The spring forceis adjusted by rotating a temperature control knob which serves as thetemperature setting means. When the force of the expanding liquidexceeds the spring force, the diaphragm actuates the electrical switchwhich deenergizes the heater. Subsequent cooling of the water reducesthe capillary pressure, thus deactivating the electrical switch,reenergizing the heater. Adjustment of the temperature control knobvaries the spring force and thus sets the water temperature at which theheater will cycle on and off.

Although these and other types of temperature controls which employmechanical control means have gained widespread use, they possessseveral limitations. For example, these controls possess a widedifferential between the temperature at which the heater is turned onand the temperature at which the heater is turned off. Thisdifferential, or hysteresis, is largely caused by the backlash and thefriction inherent in mechanically coupled systems. A wide differentialin temperature results in user dissatisfaction because of the poorrepeatability of temperature settings. This also results in energy wastesince the user will typically increase the temperature setting in aneffort to overcome the effect of the wide differential on the pool watertemperature.

Another limitation of the prior art mechanical controls is theirtendancy to fail in a mode which results in an unsafe heater condition.A typical failure mode for these controls is a leak in the sensing bulbor capillary tube, with resultant loss of fluid pressure. This type offailure causes the heater to be energized continuously, which may damagethe heater and may expose the user to dangerously high swimming pool andspa water temperatures. To overcome these problems, a separate hightemperature sensor and safety switch is usually employed in prior artcontrols to deenergize the heater when water temperature rises above apreset maximum limit.

A further limitation of prior art controls is that they are not easilyadapted for dual temperature control. Typically it is desirable for theuser to be able to preset two different temperature settings for theheater, such as a high temperature mode for the spa, and a lowtemperature mode for the pool, with means for selecting the desiredmode. With only a single temperature setting control, the user mustadjust the control knob from one setting to the other to change modes.This results in poor temperature repeatability due to the wide controldifferential as described heretofore. Alternatively, two entirelyseparate controls may be employed, which requires duplicating thesensing bulbs, capillary tubes, diaphragms and pressure operatedswitches. A separate electrical switch is then employed to select thedesired mode by connecting the appropriate pressure operated switch tothe heater controller circuit. This duplication of control systems isexpensive and cumbersome.

Still another limitation of prior art controls is that they are noteasily adapted for use in remote control applications. In many instancesit is desirable to control the heater temperature from locations otherthan the heater itself. For example, the heater may be located in anequipment area remote from the pool or spa, while the user may wish tocontrol water temperature at a location adjacent to the swimming pool.The temperature sensing bulb in the prior art controls is typicallymounted in the inlet water line of the heater, since this locationclosely approximates the spa or pool water temperature. Because thesensing bulb is connected to the temperature setting means by the metalcapillary tube, the location of the setting means is restricted to thevicinity of the heater, precluding remote control.

It is therefore an object of the present invention to provide a new andimproved temperature control for use in swimming pool and spa heaters.

It is another object of the present invention to provide a narrowdifferential temperature control for swimming pool heaters.

It is another object of the present invention to provide a swimming poolheater temperature control employing safety circuits to prevent injuryto the user or to the heater in the event of a component failure.

It is another object of the present invention to provide a swimming poolheater temperature control easily adapted for use with multiple presettemperature settings.

It is still another object of the present invention to provide aswimming pool heater temperature control easily adapted for remotecontrol of swimming pool and spa heaters.

SUMMARY OF THE INVENTION

The foregoing and other objects of the invention are accomplished by anelectronic temperature control circuit which employs a thermistor forsensing water temperature entering the pool heater. The control circuitemploys amplifiers to permit controlling the heater to maintainessentially constant water temperature within a very narrowdifferential. Two desired temperatures may be preset by either of twocalibrated potentiometers. A three-position temperature selector switchis employed to select from either of the two preset potentiometers. Thethird position of the switch is used to turn the heater off.

The control circuit is configured to permit connection of a second setof temperature setting and selection controls which may be locatedremotely from the heater. Thus a second three-position temperatureselector switch and a second set of temperature setting potentiometersmay be used for remote control of the heater. Means are also providedfor visual indication of heater operation by use of an indicator lamp.

The electronic temperature control of the present invention furtherincludes safety circuits which limit the water temperature to apredetermined maximum allowable temperature, regardless of the settingsof the temperature selector switch or the temperature settingpotentiometers.

Additional safety circuits are employed to turn off the heater in theevent of a failure of the thermistor temperature sensor.

Other objects, features and advantages of the invention will becomeapparent by reference to the specification taken in conjunction with thedrawings in which like elements are referred to by like referencedesignations throughout the several views.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of the electronic temperature controlcircuit constructed in accordance with the invention;

FIG. 2 is a circuit diagram of a remote control temperature selectorcircuit which may be used in conjunction with the invention as shown inFIG. 1;

FIG. 3 is a circuit diagram of a remote control temperature setting andselector circuit which may be used in conjunction with the invention asshown in FIG. 1;

FIG. 4 is a front view of a control panel which may be used inconjunction with the invention as shown in FIG. 1; and

FIG. 5 is a front view of a remote control panel which may be used inconjunction with the circuit as shown in FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1 there is shown an electronic temperature controlcircuit 10 constructed in accordance with the invention. The circuitreceives its power from an input voltage source connected at terminals12 and 14. Typically, this power source is derived from a step-downtransformer 16 which converts a conventional AC power source 18 to a lowvoltage level, typically 24 volts AC, which appears at terminals 12 and14. This low voltage AC source is half wave rectified by a diode 20connected as shown between terminal 14 and a ground terminal 22 and isfurther filtered by the RC network comprised of resistor 24 andcapacitor 26. The components of this filter network are chosen so that afirst DC supply voltage appears at terminal 28 and is typically twelvevolts. This first DC supply voltage is further reduced and regulated bya parallel combination of resistors 30 and 32 and by a zener voltageregulator 34, creating a second regulated DC supply voltage at terminal36 which is typically six volts. The first supply voltage at theterminal 28 is used to power operational amplifiers 38 and 40 of thecircuit 10. The second supply voltage at the terminal 36 is used topower sensor and bias networks of the circuit 10.

The control circuit 10 uses a bead thermistor 42 as the watertemperature sensing element. The thermistor 42 is housed in a suitablewaterproof enclosure which is placed in the water inlet line of theswimming pool or spa heater. This position is chosen for the sensor 42because the heater inlet water temperature is a close approximation ofthe swimming pool or spa water temperature. The thermistor 42 is aconventional negative temperature coefficient resistor whereby itsresistance decreases as its temperature increases. The sensor 42typically has a 10 kilohm resistance value at a temperature of 77° F.(25° C.) and a 3.6 kilohm resistance value at a temperature of 122° F.(50° C.). The thermistor 42 is connected to the circuit 10 at terminals44 and 46. In this position it is connected in series with a resistor 48to form a voltage divider which is in turn connected to the six voltregulated supply. As a result of the voltage divider circuit, thevoltage appearing at the terminal 44 varies with the temperature of thesensor 42 in a relatively linear fashion over the temperature range ofinterest for swimming pool and spa heaters, typically between 68° F.(20° C.) and 113° F. (45° C.). The voltage at the terminal 44 decreasesas the sensor 42 temperature increases.

The voltage at the terminal 44 is connected through a series resistor 50to the noninverting input terminal 52 of the operational amplifier 38.The amplifier 38 represents one-half of a dual operational amplifierintegrated circuit which includes the amplifier 38 and amplifier 40.These amplifiers receive their power and ground return from the twelvevolt supply through terminals 54 and 56. When the voltage at thenoninverting input terminal 52 exceeds the voltage of the invertinginput terminal 58 of the amplifier 38, the voltage at the outputterminal 60 of the amplifier 38 increases toward the positive supplyvoltage. Resistor 62 connected between the output terminal 60 and thenoninverting input terminal 52 of the amplifier 38 acts as regenerativepositive feedback to insure that the output 60 increases to the positiveoutput extreme of the amplifier 38, which is approximately ten volts.

The output terminal 60 is also connected to the emitter of an NPNtransistor 64. The base of transistor 64 is connected to the six voltsupply through a resistor 66. When the signal at the output terminal 60of the amplifier 38 is at its positive extreme, the base-emitterjunction of the transistor 64 is reverse biased and transistor 64 is cutoff.

The collector of transistor 64 is connected to the base of an NPNtransistor 68. The collector of transistor 68 is in turn connected tothe positive terminal 70 of a full wave bridge rectifier consisting ofdiodes 72, 74, 76, and 78. The emitter of transistor 68 is connected bya diode 80 to the negative terminal 82 of the full wave bridgerectifier. The input terminals 84 and 86 of the full wave bridgerectifier are connected between the main terminal 88 and the gate of aTRIAC 90.

Transistor 68 is biased to the conducting state by a resistor 92connected between the collector and the base of transistor 68. Theconduction of transistor 68 provides a low impedance path between theinput terminals 84 and 86 of the full wave bridge rectifier, providinggate bias to the TRIAC 90 and causing it to conduct. The TRIAC 90 isconnected in series between the twenty four volt AC input voltage sourceappearing at the terminals 12 and 14 and a load 94 which is connected atterminals 96 and 98. Therefore, conduction of the TRIAC 90 appliestwenty-four volts AC to the load 94. The load 94 represents a typicalheater controller for operating swimming pool and spa heaters such as anelectrically operated gas valve for controlling a gas fired heater.

The operation of the circuits discussed thus far may be summarized asfollows. When inverting input terminal 58 is more positive thannoninverting input terminal 52 of amplifier 38, the amplifier outputterminal 60 switches to a minimum output voltage level of approximatelyone-half volt. This causes the base-emitter junction of transistor 64 tobe forward biased through the resistor 66, causing transistor 64 toconduct. This conduction in turn biases the base voltage of transistor68 to a level approximately one volt above ground terminal 22. Thisvoltage level, aided by the DC offset voltage produced at the emitter oftransistor 68 by the forward voltage of the diode 80, ensures that thebase-emitter junction of transistor 68 is reverse biased. Transistor 68is thus turned off, eliminating current flow through the full wavebridge, and causing TRIAC 90 to remain in the nonconducting state.Accordingly the heater controller 94, and thus the heater, isdeenergized.

The resistor 100 connected between the gate and the main terminal 102 ofthe TRIAC 90 provides a path for leakage currents to bypass the TRIAC 90gate. Resistor 104 and capacitor 106 form a conventional RC snubbernetwork across the TRIAC 90 to minimize transient induced falsetriggering. A resistor 108 is connected in parallel with the load 94 tobypass a portion of the leakage current produced by the RC snubbernetwork components 104 and 106, and which would otherwise flow throughthe load 94 when the TRIAC 90 is nonconducting.

Returning to the amplifier 38, when the noninverting input terminal 52of amplifier 38 is more positive than the inverting input terminal 58,the amplifier output terminal 60 switches to the positive voltageextreme of approximately ten volts, turning off transistor 64 andthereby permitting transistor 68 to conduct. This causes current flowthrough the full wave bridge rectifier terminals 84 and 86 and switchesTRIAC 90 into conduction. Accordingly, the heater controller 94 isenergized, turning the heater on. From the foregoing discussion it isseen that the voltage differential at the input terminals 52 and 58 ofthe amplifier 38 controls the condition of the heater.

Circuit 10 also includes two temperature setting potentiometers 110 and112. Each of these potentiometers is connected in parallel across thesix volt DC supply in such a manner that full counter clockwise rotationof the potentiometer shaft corresponds to a six volt output levelappearing at the wiper of the potentiometer. This position of the shaftalso corresponds to the minimum heater temperature setting point.Conversely, full clockwise shaft rotation corresponds to zero voltsoutput and to the maximum temperature setting. The potentiometers 110and 112 are constructed with mechanical stops at each extreme of shaftrotation to prevent the wiper from disengaging from the resistanceelement, a condition which would produce erratic temperature settings.

The wipers of the potentiometers 110 and 112 are connected to thecontacts 114 and 116 respectively of a three position temperatureselector switch 118. Switch 118 is used to select the signal from eitherpotentiometer 110 or 112 for temperature control. Thus in position 116the switch 118 selects the temperature as set on potentiometer 112, andin position 114, the switch 118 selects the temperature setting onpotentiometer 110 for heater temperature control. The center position120 of switch 118 represents a heater "off" position.

The signal selected by the switch 118 appears at switch terminal 122 andis connected through a switch 124 to the noninverting input terminal 126of operational amplifier 40. The output terminal 128 of amplifier 40 isconnected to the inverting input terminal 58 of amplifier 38 through animpedance matching and bias network comprising a shunt resistor 130, aseries resistor 132 and a voltage divider comprising resistors 134 and136. The values for this resistor network 130, 132, 134, and 136 arechosen so that when the voltage at output terminal 128 of amplifier 40is at zero volts, the voltage appearing at the inverting input terminal58 of amplifier 38 is equal in value to the voltage appearing at thenoninverting input terminal 52 of amplifier 38 when the sensor 42 is ata temperature of 107° F. (41.7° C.). Similarly, when the voltage atoutput terminal 128 of amplifier 40 is at six volts, the values of theresistor network 130, 132, 134, and 136 produce a voltage at theinverting input terminal 58 of amplifier 38 that corresponds to thevoltage appearing at the noninverting input terminal 52 when sensor 42is at a temperature of 70° F. (21.1° C.).

The output voltage from the wiper of one of the potentiometers 110 or112 is selected by switch 118 and applied to the noninverting inputterminal 126 of amplifier 40. The gain and offset voltage of amplifier40 are determined respectively by the values of the resistor 138 and thevoltage divider comprising resistors 140 and 142 connected to theinverting input terminal 144 of amplifier 40. The values of theresistors 138, 140, and 142 are chosen so that full counter clockwiserotation of the shaft of the selected potentiometer, corresponding to aminimum desired temperature setting of 70° F. (21.1° C.) produces sixvolts at the output terminal 128 of amplifier 40. Correspondingly,clockwise rotation of the potentiometer shaft to a position representing107° F. (41.7° C.) results in zero volts at output terminal 128. Furtherclockwise rotation of the potentiometer shaft can produce no furtherdecrease in the voltage at output terminal 128, and thus has the effectof limiting the maximum available temperature set point to 107° F.(41.7° C.). This maximum temperature value has been chosen to preventphysical injury to swimming pool or spa users which might occur fromexposure to excessive water temperature.

The operation of the entire circuit 10 can now be summarized as follows.The user presets two desired temperature settings by means of thepotentiometers 110 and 112. These settings might represent a hightemperature mode for heating a spa, and a low temperature mode forheating a pool. The user then selects the desired mode by placing theswitch 118 in either position 116 or 114. The selected presetpotentiometer signal causes a voltage to appear at the inverting inputterminal 58 of amplifier 38 which corresponds to the voltage which willappear at the noninverting input terminal 52 of amplifier 38 when thetemperature of sensor 42 is at the desired preset temperature. Assumenow that the water flowing through the heater is cooler than thetemperature set point. In this condition the voltage at the invertinginput terminal 58 is less than the voltage at the noninverting inputterminal 52 of amplifier 38 which, as described heretofore, results inTRIAC 90 energizing the heater. Accordingly the water temperatureincreases, and this increase is sensed by the thermistor 42. When thetemperature equals the temperature set point, the voltages at the inputterminals 52 and 58 of the amplifier 38 are in balance and the heater isdeenergized. The typical temperature differential for the controlcircuit 10 is 1.5° F. (0.83° C.) or less, as compared to 4° F. (2.2° C.)for prior art mechanical temperature controls.

Assume now that the temperature selector switch 118 is placed in theheater "off" position 120. The noninverting input terminal 126 of theamplifier 40 is raised to a highly positive voltage by a resistor 146which is connected between the input terminal 126 and the twelve voltsupply. This results in a voltage appearing at the inverting inputterminal 58 of the amplifier 38 which corresponds to a temperature setpoint greatly below 70° F. (21.1° C.). The result is that the heater isturned off.

The temperature control circuit 10 of FIG. 1 includes a number ofadditional safety circuits to prevent the heater from being continuouslyenergized in the event of a failure of the sensor 42, such as mightoccur in some heater control circuits if the sensor or any of its leadsfail electrically. In the event of an open sensing circuit, the voltageat the terminal 44 attempts to rise toward the six volt supply throughthe series resistor 48. The voltage at the terminal 44 as mentionedheretofore is coupled to the noninverting input terminal 52 of theamplifier 38 by the resistor 50. This voltage at the input terminal 52is clamped by diode 148 which is in turn connected to a voltage dividerconsisting of resistors 150 and 152. The values of the resistors 150 and152 are chosen so that the voltage at the input terminal 52 is limitedto a maximum of approximately 3.8 volts, which corresponds to a sensor42 temperature below the lowest anticipated regulating temperature of70° F. (21.1° C.). In addition, as the voltage at the terminal 44attempts to rise due to a sensor failure, the base-emitter voltage of anNPN transistor 154 becomes forward biased by a resistor 156. The emitterand the collector of transistor 154 are connected to the inverting inputterminal 58 of amplifier 38 and the six volt supply respectively, sothat when the transistor 154 conducts, the voltage at input terminal 58is raised to six volts. Accordingly, since the voltage at the inputterminal 58 (six volts) exceeds the voltage at the input terminal 52(three and eight tenths volts), the heater is deenergized resulting in asafe heater condition in the event of a sensor circuit failure.

As an additional safety feature, the circuit 10 is designed so that nosingle component failure results in a voltage in excess of fifteen voltsor a current in excess of five milliamps appearing in the circuit ofsensor 42. These values are chosen to prevent a serious shock hazard tothe user in event of component failures. The current flowing through thesensor 42 is limited in the event of a failure by designing the circuit10 to maximize the resistance of each of the resistors 48, 156, and 50which are in series with the sensor 42. These resistors are chosen to besufficiently high in resistance to limit the current through the sensor42 to less than five milliamps at any achievable input supply voltage,regardless of a failure in the power supply circuit.

The voltage appearing at the sensor 42 is limited to a maximum offifteen volts in the event of any single component failure in thefollowing manner. The voltage to the sensor 42 is normally furnished bythe six volt supply voltage appearing at the terminal 36. This voltageis limited by the zener diode 34. In the event of the zener diode 34failing in an electrically open condition, the voltage at the terminal36 is further limited to a voltage level less than 15 volts by a seriesof redundant voltage dividers which are a natural consequence of thebias networks used for the amplifier circuits described heretofore.Thus, the voltage of the terminal 36 is supplied from the 24 volt ACsource at the terminal 12 through the resistor 24 in series with theparallel combination of the resistor 30 and the resistor 32. The voltageat the terminal 36 is held below 15 volts by the shunting effect ofresistors connected between the terminal 36 and the power supply ground22. These shunt resistors include the combination of the resistor 134 inseries with the resistor 136, the resistor 150 in series with theresistor 152, and the two temperature setting potentiometers 110 and112.

The circuit 10 as shown in FIG. 1 also includes certain filteringelements which were not mentioned in the above description of thecircuit. Thus, capacitors 158, 160, 162, and 164 are all provided asbypass filters to eliminate transient noise from false triggering thecircuit. A diode 166 is also provided between the load terminal 98 andthe power supply ground 22 to clamp negative voltage transients whichmay be generated by the heater controller 94. The heater controller 94is typically an inductive load such as a solenoid operated gas valve.

A typical control panel for use with the temperature control circuit 10is shown in FIG. 2. This control panel is typically mounted to theheater enclosure. In this figure is shown a control panel 168 formounting the three position temperature selecting switch 118 and the twotemperature setting potentiometers 110 and 112. Each temperature settingpotentiometer 110 and 112 may be provided with a graduated dial 170 and172 respectively to indicate the minimum and maximum temperaturesettings. An additional feature may be added to mechanically limit themaximum temperature setting of the potentiometers 110 and 112 to asetting below the maximum of 107° F. (41.7° C.) provided by the circuit10 as described heretofore. This mechanical temperature limit isimplemented by using an adjustable mechanical stop 174. This stop 174 isformed of a thin metal disc rotatably mounted on the shaft of thepotentiometer 110 underneath potentiometer knob 176. The stop 174 has anupstanding projection 178 which is designed to interfere with therotation of the knob 176 and prevent further rotation. The position ofthe projection 178 is user adjusted by rotating the stop 174 and thentightening a locking screw 180 which is located within a slot 182 of thestop 174. Thus the knob 176 can be set at the highest desiredtemperature and the stop 174 rotated so that projection 178 justinterferes with the knob 176. At this setting the screw 180 istightened. The stop 174 now limits the maximum temperature that can beset. Of course, a similar adjustable mechanical stop can be provided forthe potentiometer 112.

The circuit 10 of FIG. 1 is additionally provided with a connector 184to permit connection of a remote control circuit such as shown in FIG. 3for remote control of the temperature selecting switch function. Aconnector 184' shown in FIG. 3 is designed to mate with the connector184 of FIG. 1, wherein connector terminals 186, 188, 190, 192, 194, and196 mate with terminals 186', 188', 190', 192', 194', and 196'respectively. When connector 184' is mated with connector 184, and theswitch 124 of the circuit 10 is placed in position 198, which is theremote control position, the operation of the circuit is as follows. Byplacing switch 124 in position 198, the temperature selector switch 118of circuit 10 is disconnected and its function of selectingpotentiometer 110 or 112 or the "off" position is now accomplished by aswitch 200 in FIG. 3 which may be remotely located from the heater. Inaddition, a light emitting diode 202 is provided at the remote locationto give a visual indication of when the heater is energized. The lightemitting diode 202 is connected through terminals 186, 186', 196, and196' and by series current limiting resistor 204 to the output terminal60 of the amplifier 38. When the output terminal 60 switches to apositive voltage, which is the condition when the heater is energized, acurrent is supplied through the resistor 204 to light the diode 202 as aheater "on" indication. When the output terminal 60 of the amplifier 38switches to a low voltage condition, which is heater "off," the diode202 is extinguished. A typical mounting panel 206 for the remote controlcircuit of FIG. 3 is shown in FIG. 4, and includes provisions formounting the remote temperature selecting switch 200 and the lightemitting diode 202.

An alternative remote control circuit is shown in FIG. 5. This circuitincludes not only the remote control of the temperature selecting switchfunction but also remote control of the functions of the temperaturesetting potentiometers. In a manner analogous to the discussion of thecircuit of FIG. 3, connector 184" of FIG. 5 is provided to mate with theconnector 184 of FIG. 1. By placing the switch 124 in the remote controlposition 198, the circuit of FIG. 5 replaces the functions of the switch118 and the potentiometers 110 and 112 of circuit 10. Referring to FIG.5, a switch 218 performs the temperature selecting function betweenpotentiometer 228 and potentiometer 230 which are used as describedheretofore for setting two preset temperatures. Light emitting diode 232is included to act as a heater status indicator in a manner analogous tothe diode 202 of FIG. 3. A typical control panel for the circuit of FIG.5 would be similar to the panel 168 as shown in FIG. 2, with theaddition of the light emitting diode 232. It should be noted that thefunction of switch 124 described above may be accomplished by the properplacement of a jumper wire in the circuit 10, as opposed to the use ofan actual switch.

In a preferred embodiment of the invention shown in FIG. 1, componentsof the following values may be used:

    ______________________________________                                        REFERENCE                                                                     DESIGNATION   VALUE                                                           ______________________________________                                        20            1N4001                                                          24            220 Ohms                                                        26            220 Microfarads                                                 30, 32        620 Ohms                                                        34            1N5232                                                          38, 40        LM2904, National Semiconductor                                  48            8.25 Kilohms                                                    50, 92, 156   10.0 Kilohms                                                    62            2.7 Megohms                                                     64, 68        MPSA05, Motorola Semiconductor                                  66            15 Kilohms                                                      72, 74, 76, 78,                                                                             1N914                                                           80, 148, 166                                                                  90            Q2010L4, Teccor Electronics                                     100           240 Ohms                                                        104           100 Ohms                                                        106           0.05 Microfarads                                                108           2.2 Kilohms                                                     110, 112      10 Kilohm Potentiometers                                        130           1.0 Kilohm                                                      132           43.2 Kilohms                                                    134           26.1 Kilohms                                                    136           23.7 Kilohms                                                    138           14.7 Kilohms                                                    140           7.15 Kilohms                                                    142           91 Kilohms                                                      146           1.0 Megohms                                                     150           2.7 Kilohms                                                     152           3.9 Kilohms                                                     154           MPS5172, Motorola Semiconductor                                 158, 160, 162,                                                                              0.1 Microfarads                                                 164                                                                           ______________________________________                                    

While the invention as disclosed and a particular embodiment isdescribed in detail, it is not intended that the invention be limitedsolely to this embodiment. Many modifications will occur to thoseskilled in the art which are within the spirit and scope of theinvention. It is thus intended that the invention be limited in scopeonly by the appended claims.

What is claimed is:
 1. An electronic temperature control for use withswimming pool and spa water heaters comprising:means for sensing thetemperature of the water to be heated including a thermistor; means forgenerating a first electrical signal proportional to the sensedtemperature including means for deriving a voltage across the thermistorfrom a power supply; means for generating a second electrical signalproportional to a desired water temperature; means for comparing thesensed temperature with the desired temperature setting byelectronically comparing the first electrical signal with the secondelectrical signal; and means for limiting the voltage appearing acrossthe thermistor to less than the voltage appearing at the power supply inthe event of a single component failure in the electronic temperaturecontrol, including a first current limiting element connected in serieswith the thermistor to form a series circuit, a second plurality ofcurrent limiting elements connected in series between the series circuitand the power supply, and a third plurality of current limiting elementsconnected in parallel with the series circuit.
 2. The temperaturecontrol of claim 1 in which the means for limiting the voltage appearingacross the thermistor further includes a zener diode connected inparallel with the series circuit.
 3. The temperature control of claim 1in which the voltage appearing across the thermistor is limited to amaximum of 15 volts in the event of a single component failure in theelectronic temperature control.